Various types of suspension systems have been tried on the front wheel of bicycles. Most prior systems "dive" upon braking, meaning that the front of the bicycle suddenly moves downward upon application of the front brake. This phenomenon is known as brake dive. It is a problem on bicycles because their front suspensions usually have only about five cm of travel (wheel movement). Brake dive can use up over half of an already short travel, leaving only two cm or so of travel for bump absorption.
Brake dive is due to a combination of braking force pushing horizontally backwards on the front wheel, and weight transfer to the front wheel. Because the rider of a bicycle sits so high, when braking there is a large transfer of weight to the front wheel which tends to compress the front suspension. In addition, on most prior suspensions the horizontal braking force on the front tire increases the compression, resulting in excessive brake dive.
A related problem has to do with the path that the front wheel axle follows as the suspension moves. The most common type of front bicycle suspension is a telescopic fork where tubes slideably engage to provide suspension motion. This type of fork is almost universally used on motorcycles. Telescopic forks provide an axle path which follows a straight line with respect to the frame of the vehicle. The axle moves up and back as the suspension is compressed. While this motion allows the wheel to move away from large bumps, the backward braking force can push the axle back, and thus upward. As a result, telescopic forks dive. Another problem with telescopic suspensions is that they also tend to suffer from high sliding friction, which causes harshness.
Another type of suspension is the trailing link, which might be called "knee action" because of its similarity with the human leg. There is an upper leg which is rigidly attached to the bicycle frame, except for steering action. A pivot or knee, at the end of the upper leg, allows the lower leg or trailing link to swing in an arc. The wheel axle is attached, directly or indirectly to the trailing link. The axle arcs back and then up as the suspension compresses, a motion which improves absorption of large bumps by allowing the wheel to move back and away from the bump. This helps avoid flat tires caused by pinching the tube between the wheel rim and a rock. Unfortunately, the axle path of a trailing link suspension worsens the brake dive problem because the axle path is almost directly in line with the combined forces of weight transfer and braking. The only prior trailing link bicycle suspension used a disk brake, where the brake caliper was mounted on a four bar linkage arranged to reduce dive. It was thought that a hub brake, such as a disk brake, was necessary to make a working trailing link suspension. Hub brakes are heavier and more expensive than rim brakes, which are normally used on bicycles.
The third major category of front suspensions is the leading link type. This is similar to a trailing link suspension, except reversed front to back. A pivoting link extends from a fixed leg forward to the wheel axle. The main problem with this arrangement is that the axle now arcs in an undesireable manner. The axle moves upward and then back, relative to the bicycle frame. This action makes the suspension harsher on sharp bumps. It also causes undesireable changes in steering geometry. Because the axle moves farther from the steering axis when initially compressed, steering stability is lessened on bumps. Also, it was thought that a hub brake was necessary to get anti-dive action on a leading link suspensions.
Some leading link suspensions use an arrangement with two leading links on each side, mounted well above the axle, and angled so that the axle path mimics that of a trailing link suspension. The problem with these designs is that brake dive is worsened, and it cannot easily be remedied.
In summary, most prior art suspensions are "pro-dive" in that braking force tends to compress the suspension. No prior suspension using rim brakes has correctly balanced the forces at work to provide "anti-dive" action. The only anti-dive suspensions to date have used hub brakes, which are expensive and heavy.